1. Field of the Invention
The present invention relates to ships and watercrafts having improved efficiency and seakeeping from underwater submerged displacement hull(s) attached to and part of a vessel that operates at sea level.
2. Background of the Invention
In recent years interest in the use of small waterplane area ships (SWAS vessels) has substantially increased because such vessels have improved hydrodynamic stability, low water resistance and minimal ship motion. Generally such vessels have at least one waterline located below its design draft with a waterplane area that is significantly larger than the waterplane area at its design draft. One form of such vessel is known as a small waterplane area twin hull vessel (a SWATH vessel) which generally consists of two submerged hulls, originally formed of uniform cross-section, connected to a work platform or upper hull by elongated struts which have a cross-sectional area along any given waterplane area that is substantially smaller than a waterplane area cross-section of the submerged hulls. Thus, at the design waterline such vessels have a small waterplane area.
The interest in such vessels has increased in large part because of the development work conducted by Pacific Marine Supply Co., Ltd. A variety of such vessels have been produced using twin submerged hulls or a plurality of submerged hulls, such as shown, for example, in U.S. Pat. No. 5,433,161. In the course of the development work for these vessels, further improvements were made and a so-called Mid-Foil SWAS vessel was developed, as disclosed in U.S. Pat. No. 5,794,558. Such vessels use a submerged underwater displacement hull or lifting body to provide lift to the craft in conjunction with any other parts of the vessel which generate lift. The lifting body differs from a hydrofoil in that the enclosed volume of the lifting body provides significant displacement or buoyant lift as well as hydrodynamic lift whereas the lift of a hydrofoil is dominated by only hydrodynamic lift. In the course of continuing development work, the particular shape of such lifting bodies was studied in detail in order to improve their performance and adapt and integrate their use to a wide range of marine craft.
More specifically, as disclosed in U.S. Pat. No. 6,263,819, it was found that the submerged bodies of marine vessels, when operated at shallow submergence depths, such as is the case for SWAS and Mid-Foil vessels, can be adversely effected by the displacement of the free water surface caused by the body's volume and dynamic flow effects. The interaction of that displacement of the free surface relative to the body's shape had not been adequately accounted for in the prior art structures. It is believed that this inadequacy of existing prior art submerged bodies for marine vessels is the result of the fact that submerged and semi-submerged marine vessels have historically been designed to operate at great depths relative to their underwater body thickness, as with submarines or hydrofoils.
A typical submarine is essentially a body of revolution-shaped hull which has three dimensional waterflow about it, but which is designed to operate normally several hull diameters or more below the free water surface. Thus, the displacement of the free surface of the water by operation of the hull at such depths is minimal and does not effect the operation of the body. On the other hand, hydrofoils are simply submerged wings with predominately two-dimensional flow and are designed typically to produce dynamic lift as opposed to buoyant or hydrostatic lift.
The displacement of water at the free surface by a submerged body is detrimental to a marine vessel's hydrodynamic performance with the impact varying as a function of the body's shape, submergence depth, speed and trim. For example, the free surface effects can significantly reduce lift in the body or even cause negative lift (also referred to as sinkage) to occur. Resistance to movement through the water by free surface effects is generally greater than if the submerged hull were operating at great depths; and pitch movements caused by the displacement of the free water surface vary with speed and create craft instability. With the advent in recent years of marine vehicles (such as the SWAS, SWATH, and Mid-Foil vessels) which use a shallowly submerged body the detrimental effects of free surface water displacement on submerged hulls has been recognized.
Prior to the invention as disclosed in U.S. Pat. No. 4,263,819, submerged displacement watercraft hull body shapes were generally cylindrical or tear-drop shaped bodies of revolution. The simplest variations are bodies with generally elliptical cross-sections, such as are shown, for example, in U.S. Pat. No. 4,919,063 or 5,433,161. Others were simply shaped in a manner similar to an airplane wing, as shown for example, in U.S. Pat. No. 3,347,197. On the other hand, hydrofoil dynamic lift shapes are generally thin-foils with little or no, buoyancy and symmetric foil sections having straight leading and trailing edges. In plan these foils are generally straight, or are swept forward or rearwardly and/or are trapezoidal in shape. Additionally, they can have dihedral or anhedral canting from the horizontal. It was found that the performance of vessels using these shapes is adversely effected by the displacement of the free surface of the water above the bodies during operation of the vessel.
According to teaching of U.S. Pat. No. 6,263,819 (hereinafter the “'819 patent”), a low drag underwater submerged displacement hull is defined from two parabolic shapes. The periphery of the hull when viewed in plan is symmetrical and defined by a first parabolic form (or parabolic equation) with the form defining the leading edge of the hull. The longitudinal cross-section of the hull is formed of foil shaped cross-sections which are defined as cambered parabolic foils having a low drag foil shape and providing a generally parabolic nose for the hull. Generally, each longitudinal cross-section of the hull parallel to the longitudinal or fore and aft axis of the hull has a symmetrical cambered parabolic foil shape with the cross-section along the longitudinal axis of the hull having the maximum thickness and the cross-section furthest from the centerline of the hull having the minimum thickness. In plan, the hull has a stern or trailing edge which is defined by either a straight line, a parabolic line, or a straight line fared near its ends to the side edges of the plan parabola shape.
In another embodiment the hull shape is a parabolic body of revolution. In a third embodiment the hull also has a foil shape in longitudinal cross-section which is essentially formed by a parabolic body of revolution cut in half and separated by a uniform midships section, whose longitudinal cross-sections are uniform in shape and correspond to the parabolic shape of the body of revolution.
These body shapes have benign pressure gradients and small stagnation points over the body which make the bodies less sensitive to changes in the body angle of attack relative to the flow so that they are less effected by free water surface disturbance. Parabolic foil embodiments have high Block coefficients which maximize their volume to surface area relationship with the result that they have less frictional drag because of reduced wetted surface area, less structure and thus less cost. With higher Block coefficients, such as the 60–70% coefficients achieved with the lifting bodies of the '819 patent, the volume of the foil relative to its surface area is maximized and, as a result, the foils provide greater buoyancy for the same surface area as compared to the prior art.
Because of their high Block coefficient, high displacements can be achieved with hulls having relatively short bodies. This allows these bodies to operate at high Froude numbers, preferably in excess of 1. This in turn results in less wave making drag and less friction drag from a thinner boundary layer. Wakes formed by these bodies are very uniform and result in minimal disturbance beyond the trailing edge to appendages bodies, or propulsers positioned at the trailing edge or stern. The symmetrical parabolic foils, at critical design submergence depths, displace the free surface of the water in a manner which reduces the pressure coefficient on the bodies and allow higher incipient cavitation speeds. Their dynamic lift can then be varied as a function of camber (i.e. variation of the surface location from the design parabola), submergence, speed and angle of attack. As a result, optimization of lift characteristics for a given craft design speed and draft can be achieved. Further, dynamic lift of these bodies can be varied by the use of integrated trailing edge flaps, which will mitigate appendage drag of non-integral foil stabilizers.
It has been found that the symmetric lifting bodies of the '819 patent operate very satisfactorily for most applications, even for very large vessels of 2000 tons and up. However, it is advantageous to have lifting bodies which are smaller relative to the length of the ship and capable of being positioned outboard of the watercraft hull. Therefore, further development of the lifting bodies of the '819 patent has occurred, particularly for use with monohull vessels.
The symmetrical lifting bodies as disclosed in the '819 patent were primarily used generally directly under the hull. However, if the lifting body is located further from the center of gravity of the ship, it not only can provide lift but greater dynamic control as a result of maximizing dynamic moment. In addition, it has been found useful to tailor the shape of the lifting body to conform to the hull it is used with as well as to accommodate flows under the hull caused by the hull or other underwater structures. It also has been found that while large monohull vessels have very good seakeeping ability, the use of the tailored asymmetric lifting bodies of the present invention with such hulls greatly increase their seakeeping abilities.
It is an object of the present invention to provide a submerged lifting body which can be employed on various marine vessels to maximize performance of the vessel by creating a high lift to drag ratio (L/D), i.e., low drag, at operational speed, while increasing dynamic control.
Another object of the present invention is to provide a submerged lifting body for use on various marine vessels which improves performance of the vessel at operational speed while creating a dynamically stable vessel.
Yet another object of the present invention is to provide submerged lifting bodies for use on various marine vessels which can increase the efficiency of these vessels by reducing hydrodynamic drag.
A further object of the present invention is to adapt these improved submerged lifting bodies to a variety of watercraft (monohulls, catamarans, trimarans, swath, semi-swath, planing and displacement vessels) by optimizing their shape, size, number and location.
Another object of the present invention is to provide submerged lifting bodies for use on various marine vessels that are shaped to reduce the possibility of being damaged when docking or coming alongside another structure.
Yet another object of the present invention is to provide submerged lifting bodies for use on various massive vessels that reduce the wave making and slamming of a vessel.
Yet another object of the present invention is to provide submerged lifting bodies for use on various marine vessels that improve the seakeeping by reducing the vessel's motions while at rest as well as while underway.
Still another object of the invention is to provide submerged lifting bodies for use on various marine vessels that are shaped to result in improved flow to an integrated propulsor yielding high propulsive efficiency.